U.S. patent number 6,465,910 [Application Number 09/782,402] was granted by the patent office on 2002-10-15 for system for providing assured power to a critical load.
This patent grant is currently assigned to UTC Fuel Cells, LLC. Invention is credited to Francis A. Fragola, Jr., Herbert C. Healy, Ricky M. Ross, Douglas Gibbons Young.
United States Patent |
6,465,910 |
Young , et al. |
October 15, 2002 |
System for providing assured power to a critical load
Abstract
A power system (8) is provided for economically supplying
uninterrupted electrical power to one or more critical loads (14).
One or more fuel cell power plants (18) provide one substantially
continuous source of power, and a utility grid (10) provides
another source of power. The fuel cell power plants (18) are
adapted to be, and are, normally substantially continuously
connected and providing power to, the critical load(s) (14). A
rapidly-acting static switch (19) selectively connects and
disconnects the grid power supply (10) to the critical load(s) (14)
and with the fuel cell power plant(s) (18). A switch controller
(49, 45) controls the state of the static switch (19) to connect
the grid power source (10) with the critical load(s) (14) and the
fuel cell power plant(s) (18) during normal operation of the grid
(10), and to rapidly (less than 4 ms) disconnect the grid power
source (10) from the load(s) (14) and fuel cell power plant(s) (18)
when operation of the grid (10) deviates from normal beyond a
limit.
Inventors: |
Young; Douglas Gibbons
(Suffield, CT), Healy; Herbert C. (Windsor, CT), Fragola,
Jr.; Francis A. (Wallingford, CT), Ross; Ricky M. (South
Windsor, CT) |
Assignee: |
UTC Fuel Cells, LLC (South
Windsor, CT)
|
Family
ID: |
25125937 |
Appl.
No.: |
09/782,402 |
Filed: |
February 13, 2001 |
Current U.S.
Class: |
307/64; 307/43;
307/80; 307/44; 307/70; 307/45 |
Current CPC
Class: |
H02J
3/32 (20130101); H02J 9/062 (20130101); H02J
2300/30 (20200101); Y02B 90/10 (20130101) |
Current International
Class: |
H02J
3/32 (20060101); H02J 3/28 (20060101); H02J
9/06 (20060101); H02J 007/00 () |
Field of
Search: |
;307/64,70,43,44,45,80 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Jackson; Stephen W.
Assistant Examiner: DeBeradinis; Robert L
Attorney, Agent or Firm: Schneeberger; Stephen A.
Claims
What is claimed is:
1. A power system (8) for providing uninterrupted electric power to
a critical load (14), comprising: a. a first power source (10)
providing sufficient power to supply the critical load (14); b. a
second power source (18) comprising at least one fuel cell power
plant (18), the second power source providing sufficient power to
supply the critical load (14) and adapted to be normally
substantially continuously connected and providing power to, the
critical load (14); c. a static switch (19) for selectively
connecting and disconnecting the first power source (10) to the
second power source (18) and (to) the critical load (14); and d. a
switch controller (49, 45 )for controlling the state of the static
switch (19) to connect the first power source (10) with the
critical load (14) and the second power source (18) during normal
operation of the first power source (10) and to rapidly disconnect
the first power source (10) from the critical load (14) and the
second power source (18)if and when operation of the first power
source (10) deviates beyond a limit from normal.
2. The power system (8) of claim 1 wherein the switch controller
(49, 45) additionally controls the state of the static switch (19)
to rapidly reconnect the first power source (10) with the critical
load (14) and the second power source (18) when the first power
source (10) returns to normal operation.
3. The power system (8) of claim 1 wherein the second power source
(18) comprises only one or more fuel cell power plants (18).
4. The power system (8) of claim 1 wherein the static switch (19)
is a solid-state device.
5. The power system (8) of claim 4 wherein the solid-state device
is a thyristor (19).
6. The power system (8) of claim 1 wherein the first power source
(10) is a utility power grid and wherein each fuel cell power plant
(18) includes a power conditioning system (PCS) for configuring
operation of the respective fuel cell (18) in a grid connected mode
or in a grid independent mode in response to mode control signals
(D1/401', D2/402'), and including a site management controller (31)
connected intermediate the switch controller (49, 45) and the power
conditioning system (PCS) and responsive to preliminary mode
signals (M1/401, M2/402) from the switch controller (49, 45) for
providing the mode control signals (D1/401', D2/402') to the fuel
cell power conditioning system (PCS), whereby the fuel cell power
plants (18) rapidly transition operation between the grid connected
and the grid independent modes.
7. The power system of claim 6 wherein the rapid disconnection of
the first power source (10) from the critical load (14) and the
second power source (18), and the rapid transitioning of operation
of the at least one fuel cell (18) between the grid connected mode
and the grid independent mode occurs within an interval of about 4
milliseconds.
8. The power system of claim 1 wherein the rapid disconnection of
the first power source (10) from the critical load (14) and the
second power source (18) occurs within an interval of about 4
milliseconds.
9. A power system (8) for providing substantially continuous
electric power to at least a critical load (14), comprising: a. a
utility grid power source (10) providing sufficient power to supply
the critical load (14); b. at least one fuel cell power plant (18)
operating substantially continuously for providing at least
sufficient power to supply the critical load (14), the at least one
fuel cell power plant (18) including a power conditioning system
(PCS) for configuring operation of the respective fuel cell (18) in
a grid connected mode or in a grid independent mode in response to
mode control signals (D1/401', D2/402'), the at least one fuel cell
power plant (18) being normally substantially continuously
connected and providing power to, the critical load (14); c. a
static switch (19) for selectively connecting and disconnecting the
grid power source (10) to the at least one fuel cell power plant
(18) and to the critical load (14); d. a switch controller (49, 45)
for controlling the state of the static switch (19) to connect the
grid power source (10) with the critical load (14) and the at least
one fuel cell power plant (18) during normal operation of the grid
power source (10) and to disconnect, within a 4 millisecond
interval, the grid power source (10) from the critical load (14)
and the at least one fuel cell power plant (18) when the grid power
source deviates beyond a limit from normal; and e. a site
management controller (31) connected between the switch controller
(49, 45) and the power conditioning system (PCS) and responsive to
preliminary mode signals (M1/401, M2/402) from the switch
controller (49, 45) for providing the mode control signals
(D1/401', D2/402') to the fuel cell power conditioning system (PCS)
to cause the at least one fuel cell power plant (18) to rapidly
transition operation, within a 4 millisecond interval, between the
grid connected mode and the grid independent mode.
10. A power system (8) for providing substantially continuous
electric power to at least a critical load (14), comprising: a. a
utility grid power source (10) providing sufficient power to supply
the critical load (14); b. at least one fuel cell power plant (18)
operating substantially continuously for providing at least
sufficient power to supply the critical load (14), the at least one
fuel cell power plant (18) including a power conditioning system
(PCS) for configuring operation of the respective fuel cell (18) in
a grid connected mode or in a grid independent mode in response to
mode control signals (D1/401', D2/402'), the at least one fuel cell
power plant (18) being normally substantially continuously
connected and providing power to, the critical load (14); c. a
static switch (19) for selectively connecting and disconnecting the
grid power source (10) to the at least one fuel cell power plant
(18) and to the critical load (14); d. a switch controller (49, 45)
for controlling the state of the static switch (19) to connect the
grid power source (10) with the critical load (14) and the at least
one fuel cell power plant (18) during normal operation of the grid
power source (10) and to disconnect, within less than an 8.3
millisecond interval, the grid power source (10) from the critical
load (14) and the at least one fuel cell power plant (18) when the
grid power source deviates beyond a limit from normal; and e. a
site management controller (31) connected with the switch
controller (49, 45) and the power conditioning system (PCS) and
responsive to the switch controller (49, 45) for providing mode
control signals (D1/401', D2/402') to the fuel cell power
conditioning system (PCS) to cause the at least one fuel cell power
plant (18) to rapidly transition operation, within less than an 8.3
millisecond interval, between the grid connected mode and the grid
independent mode.
11. The power system (8) of claim 10 wherein the at least one fuel
cell power plant (18) is caused to rapidly transition operation
between the grid connected mode and the grid independent mode in an
interval of less than about 4 milliseconds.
Description
TECHNICAL FIELD
This invention relates generally to power systems, and more
particularly to power systems for providing an assured, or
uninterruptible, supply of electrical power to one or more critical
loads. More particularly still, the invention relates to such power
systems employing fuel cells as a source of electrical power.
BACKGROUND ART
By far, the most common source of electrical power for a great
variety of loads is via the extensive power grid provided by the
various electric utilities. The electrical power available on the
utility grid is generally fairly reliable as to continuity and
adherence to established standards of voltage, frequency, phase,
etc. However, from time to time discontinuities and/or departure
from those standards do occur. If they are brief or modest, most
loads are relatively insensitive to those events. On the other
hand, there are a growing number of loads which are relatively
intolerant of even brief aberrations in the power supplied by the
utility grid, with the principal example being computers and
various types of electronic data processing devices. Even brief
interruptions in the standardized supply of electric power by the
utility grid may cause the computer to malfunction, with sometimes
costly, and always bothersome, consequences.
In defining this concern, the Computer Business Equipment
Manufacturers Association (formerly CBEMA, and now ITI) has
developed a set of Power Acceptability Curves which establish the
standards, or at least provide guidance, for determining the power
norms which will assure continued operation of those types of
loads. In that regard, a standard has been adopted indicating that
a computer can tolerate a one half cycle or 8.3 ms power
interruption. The power available on the utility grids is not
presently capable of meeting this standard on a substantially
continuous basis. Accordingly, it has been and is, necessary to
provide supplemental power sources if it is important to assure
that critical loads have a substantially continuous or
uninterrupted supply of electrical power. For purposes of this
application, a supply of power with interruptions or transfers of
no greater than 8.3 ms duration, may be referred to as being
"seamless", "substantially continuous", or "substantially
uninterrupted".
Referring to FIG. 1, there is illustrated one existing form of
uninterruptible power supply (UPS), a so-called "on-line" or
"double conversion" type, used to supply a critical load in those
instances when the utility grid supply is interrupted or is outside
of specified limits. The utility grid power supply normally appears
on conductor 110, and is passed via normally-closed contacts of a
3-pole transfer switch 112 to a rectifier 120, which supplies the
critical loads 114 via an inverter 122. However, to provide
continued power in and during those intervals when the utility grid
power is not within the specified limits, a backup battery 116 is
provided to supply immediate power of limited duration, and an
emergency electrical generator 118 is then connected to the other
contact of transfer switch 112 to follow-up with a longer term
temporary supply. To accommodate the use of battery 116 in a system
which relies on AC power for the loads 114, it is necessary to
provide the rectifier 120 to charge battery 116 and the inverter
122 to convert the DC supply from the battery to the necessary AC
supply for the loads. A high speed switch 124 connected between the
transfer switch 112 and the loads 114 operates as a bypass switch
to provide temporary power if the inverter 122 or rectifier 120
must be serviced. Because the grid and loads are not normally
directly connected, but rather the power to the loads is required
to pass through a pair of converters with the aid of the UPS
battery, this type of UPS is termed an "in-line" or "double
conversion" type. This arrangement, though effective, requires a
number of costly components that are in use only during the
intervals when the utility grid power is unsatisfactory.
Another arrangement of a power system for providing substantially
uninterrupted power to critical loads is described in PCT
application US99/10833 for "Power System", published on Nov. 25,
1999 as WO 99/60687. Referring to FIG. 2 in the present
application, the relevant portions of the invention described in
that PCT application are depicted in a very simplified, generalized
form, with elements being numbered such that their last 2 digits
are the same as their functionally equivalent counterparts in FIG.
1. The critical loads 214 receive substantially uninterrupted power
from a motor-generator 230 within an uninterruptable power system
module 231, which module also contains transfer switches,
rectifiers and inverters. Several alternative electrical power
sources are provided to maximize the continued powering of the
motor-generator 230. One such power source may be the utility grid
210. Another source may be the fuel cell generator power plant 218.
A transfer switching arrangement 212 enables one or the other of
the utility grid 210 and the fuel cell 218 to normally provide the
power to drive the motor-generator 230. This type of
uninterruptible power supply is also of the "on-line" or "double
conversion" type inasmuch as the grid is not directly connected to
the loads 214, but acts through the rectifier and inverter
converters and the flywheel and/or fuel cells to energize
motor-generator 230 which in turn provides uninterrupted power. In
fact, the fuel cell 218 is configured to operate in a grid connect
(G/C) mode with the utility grid 210 for system economy, so in grid
connected mode both the grid and the fuel cell supply the "grid"
terminals of the transfer switch. In the event of failure of the
grid supply 210, the fuel cell 218 is intended to serve as the
continuing power source for the motor-generator 230. However, in
such event, the fuel cell 218 must reconfigure from a "grid
connect" (G/C) mode of operation to a "grid independent" (G/I)
mode. The power conditioning system (PCS) portion of the fuel cell
218 includes associated inverters, switching transistors and
breakers (not shown) that effect the conversion of DC power to AC
power and that govern the fundamental G/C and G/I modes of fuel
cell operation. That mode transition (from G/C to G/I) has
typically required the fuel cell 218 and transfer switch 212 to
interrupt power generation for up to 5 seconds. Such interruption
is not "seamless", and would be of unacceptable duration for
critical computer loads 214. Accordingly, a backup flywheel power
source 216 provides immediate power of limited duration(similar to
the battery source 116 in FIG. 1) to the motor-generator 230 at
least during such mode conversions. That backup power source 216 is
a flywheel 236 driving a bi-directional AC/DC converter 238. The
converter 238 keeps the flywheel spinning during normal operation,
and discharges the flywheel 236 during backup operation. The
various transfer switches used in the transfer switching
arrangement 212 and in the uninterruptable power system module 231
may be electro-mechanical, static, or a combination thereof, and
serve to effect the various power switching functions.
While the Power System of the abovementioned PCT application may
provide a substantially uninterrupted source of power to various
critical loads and may advantageously employ fuel cells as one of
the main sources of the power, it nevertheless requires the use of
considerable additional equipment that is complex and costly. For
example, the separate motor-generator 230, and the backup power
source 216 which includes the flywheel 236/converter 238
combination, represent necessary, but expensive, components in
order to assure the degree of power continuity sought and
required.
Another type of UPS is of the "stand-by" type wherein the grid is
directly connected to the loads and a stand-by UPS remains idle,
even if connected to the loads, until a switch disconnects the grid
from the loads. An example of such a system is disclosed in U.S.
Pat. No. 6,011,324. The fuel cell and associated inverters are
normally connected to the loads, but in an idle standby mode while
the grid supplies power directly to the loads. When the grid fails,
the fuel cell is rapidly brought to full output power and a solid
state switch disconnects the grid. Here, too, a number of costly
components are in use only during the intervals when the utility
grid power is unsatisfactory. Accordingly, it is a principal object
of the present invention to provide a power system for providing a
substantially uninterrupted (seamless) supply of electric power to
critical loads in a relatively economical manner.
It is a further object to provide such a power system in which one
or more fuel cell power plant(s) are utilized to substantially
continuously supply power to the loads.
DISCLOSURE OF THE INVENTION
According to the invention, there is provided a relatively
economical and reliable power system for providing substantially
uninterrupted electric power to one or more critical loads. A first
power source, such as the utility grid, provides sufficient power
to supply the critical loads. A second power source comprising at
least one, and typically multiple, fuel cell power plants, provides
sufficient power to supply at least the critical loads. The fuel
cell power plant(s) is/are adapted to be, and is/are, substantially
continuously connected to the critical loads and are substantially
continuously providing significant power to at least the critical
loads. A static switch operates to rapidly and seamlessly connect
and disconnect the utility grid to the critical load(s) and to the
fuel cell power plant(s), for economical continuous usage of the
fuel cell power plant(s). Significant economy is realized by having
the substantially continuously operating fuel cell(s) substantially
continuously connected to the load, and normally also to the grid.
In this way, the fuel cells may continuously deliver their rated
power, with the requisite portion going to the critical loads and
any excess being delivered to non-critical loads and/or the grid.
The static switch may be one or more silicon controlled rectifiers
(SCRs), or thyristors. Solid-state switch controls operate to
rapidly switch the static switch in 4 ms, or less, to make seamless
transfers between the first and second power sources. This
switching speed is significantly faster than is obtained with
conventional line commutation of thyristors. Further control
electronics provide high-speed transitions (less than about 4 ms)
in the operating modes of the power conditioning system (PCS)
inverters associated with each of the fuel cell power plants. This
assures that the fuel cell mode transitions, heretofore normally
slow, are at a speed comparable to that of the static switch so as
to provide substantially seamless power transfers of and between
the first and second power sources. This allows continuous
productive operation of the fuel cell power plants.
The foregoing features and advantages of the present invention will
become more apparent in light of the following detailed description
of exemplary embodiments thereof as illustrated in the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a simplified schematic block diagram of one type of
uninterruptible power supply in accordance with the prior art;
FIG. 2 is a simplified schematic block diagram of an
uninterruptible power supply employing fuel cell power plants in
accordance with the prior art;
FIG. 3 is simplified schematic block diagram of a power system
employing a fuel cell power plant, static switch and site control
in accordance with the invention;
FIG. 4 is a schematic block diagram illustrating the static switch
in greater detail;
FIG. 5 is a schematic block diagram illustrating the site control
in greater detail; and
FIG. 6 is table of the operational mode states of the fuel cell(s)
in association with mode-controlling signals.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to the Drawings, FIGS. 1 and 2 depict prior types of
uninterrupted power systems as previously described in the
Background Art.
Referring to FIG. 3, there is depicted a simplified schematic block
diagram of a power system 8 in accordance with the invention. The
power system 8 is connected to utility grid bus 10, and employs one
or more fuel cell power plants 18 at a site, for supplying 3-phase
power substantially continuously to and through load contactors
(not shown), to load(s) 14, usually also at the site. For
simplicity, a "one line" diagram, or representation, is used herein
to depict the 3-phase supply lines, as well as their included
switches, etc. The grid 10, the fuel cell power plants 18, and the
load(s) 14 are interconnected and controlled through a site
management system (SMS), generally represented by the broken line
block, or grouping, 11. The load(s) 14 typically include a number
of individual customer loads, at least some of which require a
substantially continuous supply of power and are thus deemed
"critical loads". The critical loads 14 are typically computers,
control devices employing computers, and/or electronic data
processing devices. For convenience of explanation and visual
distinction, the portions of the schematic carrying the relatively
higher voltage/current/power to the load(s) 14 are bolded, in
contrast with the lower-voltage, control portions of the system
8.
The utility grid bus 10 normally provides power at 480 V.sub.AC and
60 Hz, as do the fuel cell power plants 18 via lead, or bus, 15.
Switching gear, generally designated 12, serves to interconnect the
fuel cell(s) 18, the load(s) 14 and the utility grid 10. In this
way, the fuel cells 18 are available and connected for supplying
electrical power on a full time basis to the loads 14 and/or to the
utility grid 10, for economical usage of the fuel cells. The
switching gear 12 includes a static switch module 17 for
selectively connecting and disconnecting the utility grid bus 10 to
the loads 14 and to the fuel cells 18, as will be described. The
static switch module 17 includes a 3-pole electrically operated
static switch 19 rated at 2000 amperes and capable of performing
seamless switching transfer of power in about 1/4 cycle (about 4
ms). The switching gear 12 further includes several inter-tie or
breaker switches 21, 21A, 23, 23A, and isolation switch 25, for
further selectively connecting and disconnecting the fuel cells 18,
loads 14, utility grid bus 10 and static switch module 17, relative
to one another, primarily to isolate the static switch 19 for
servicing and continue to provide power to the load(s) 14. A
secondary purpose is to allow large fault currents to flow through
breaker 23A instead of static switch 19 if such a fault in the load
14 should occur.
The fuel cell(s) 18 may be a single power plant, or multiple (i.
e., "n") plants, connected to provide power to the loads 14 and/or
to the utility grid 10. In an exemplary embodiment, there are five
fuel cell power plants 18, each being a 200 kw ONSI PC25.TM.C power
plant, for collectively providing up to 1 megawatt of power. In
addition to a fuel processor and the fuel cell stack itself, each
power plant 18 also includes a power conditioning system (PCS) that
contains a solid-state inverter which converts DC power to AC power
at the desired voltage and frequency. Control of and by the PCS
further enables conversion of the mode of operation of a fuel cell
power plant 18 from G/C to G/I, and vice versa, as will be
described in greater detail. When used in G/C mode, the variable
controlled by the PCS is power delivered (both real and reactive).
When used in the G/I mode, the variables controlled are output
voltage and frequency, and, if multiple power plants 18 are
involved, phase. The output voltage of a three-phase system is, of
course, controlled to be at a phase angle of 120.degree. between
each phase. The outputs of the several fuel cell power plants 18
are collectively joined by bus 15, which is connected through a
delta-to-wye transformer 27 and bus 15' to the switching gear 12.
The transformer 27 provides a separately derived neutral/ground
system for the load 14, and also provides isolation between the
fuel cell PCS and the load 14 and/or the utility grid bus 10.
A site supervisory control (SSC) 29 provides the operator interface
for the system 8 and may be responsible for control of the system
at a high level. The SSC 29 allows the operator to issue high level
commands such as "start", "stop", and the like. The SSC 29 may
include one or more programmable logic controllers, data
processors, computers, sensors, etc. to effect the control of the
various components and functions of the system 8. An operator
console 32 provides a display and input capability for the SSC 29.
The SSC 29 may provide limited control of switching gear 12, as
through a link 52, although principal control of that switching
gear occurs automatically by the static switch 19.
There is also provided a site management control (SMC) 31 for
providing direct control of the PCS's of the fuel cells 18, in
response to signals from the static switch module 17, as well as
the grid voltage reference signal 10' described below. The SMC 31
also may be composed of computers and associated sensors and
control circuitry. The SMC 31 may be viewed and considered as an
included portion of the SMS 11. Control bus 33 exchanges control
signals between the SMC 31 and the PCS's of fuel cells 18. Control
signals may also be exchanged between the SSC 29 and the fuel cells
18 via control bus 35, here shown in broken line. Control signals
are exchanged between the SMC 31 and the static switch module 17
via control bus 40. A voltage, or potential, transformer 37 senses
the 480 V.sub.ac grid voltage and communicates the stepped-down 120
V.sub.ac value, via bus 10', to the SMC 31 and the static switch
module 17 for the purpose of providing control signals indicative
of the grid's voltage, phase and frequency. The depicted location
and quantity of transformer(s) 37 is mainly symbolic, and it should
be understood that such transformer(s) may, alternatively, be
incorporated as part of the control circuit or module for which the
control signal is provided. A current transformer 41 senses the
load current in a power bus path 39 connected to the loads 14, and
communicates the value to the static switch module 17 via bus 43.
Similarly, current transformer 42 senses grid current and
communicates the value to the static switch module 17 via bus 44,
and voltage transformer 46 senses load voltage and transmits it to
the static switch module 17 via bus 48.
Returning to further consideration of the switching gear 12, with
reference additionally to FIG. 4, the power bus 15' from the fuel
cells 18 is connected through breaker 21 to one pole of the static
switch 19. The power bus path 39 extends from that pole of the
static switch 19 through a normally-closed isolation switch 25 to
the loads 14. The utility grid power bus 10 is extended to the
other pole of the static switch 19 through breaker 23. The breaker
switches 21 and 23 are intended to be closed during normal
operation, such that power from the fuel cells 18 and/or the
utility grid 10 may be supplied to the loads 14, assuming the
static switch 19 is closed. Similarly, assuming the power delivered
by the fuel cells 18 to the critical loads 14 is less than the
cells' entire power output, the excess power from the fuel cells 18
may be delivered through the static switch 19 to the utility grid,
or at least to customer non-critical loads (not shown) located on
the grid side of static switch 19. In fact, this is the preferred
economic mode of operation in that it maximizes the use of the fuel
cells 18 and minimizes the need for and cost of, power from grid
10.
A bypass breaker switch 21A, connected from power bus 15' to the
power bus path 39 between the loads 14 and the isolation switch 25
and being normally open, serves, when closed, to bypass breaker
switch 21 for purposes of maintenance or isolation. Similarly, a
bypass breaker switch 23A, connected from the utility grid bus 10
to the power bus path 39 between the loads 14 and the isolation
switch 25 and being normally open, serves, when closed, to bypass
breaker switch 23 and static switch 19 to supply grid power to
loads 14, in the event the static switch fails or during
maintenance or during a load fault sufficiently large to exceed the
rating of the static switch. Breakers 21, 23, and 23A are
electrically operated and are automatically controlled by the
static switch 19 to perform transfers in 5 or 6 cycles, e.g., about
80-100 ms. The breaker switch 21A and isolation switch 25 are
manual. The switches 21, 23, and 23A can also be manually
controlled by the SSC 29. Each of the switches 21, 21A, 23, 23a,
and 25 is rated 2000 amperes, and the circuit breakers have a fault
interrupting rating of 65 kaic. The general communication link 52,
shown in broken line between the switching gear 12 and the SSC 29,
serves to convey appropriate status and manual control signals
therebetween for the static switch 19 and the several breakers 21,
23, 23A, etc. Control logic 49 associated with static switch module
17, and particularly a switchgear control logic portion 49B
thereof, serves to control the several breakers and switches 21,
21A, 23, and 23A, as represented by the broken line control paths
21', 21A', 23', and 23A' extending thereto. The control logic 49 is
generally comprised of a high-speed logic portion 49A for rapidly
controlling the static switch 19, and a relatively slower-speed
portion 49B for controlling the remainder of switchgear 12.
Referring still further to FIG. 4, the static switch module 17 is
depicted in greater detail. Although the static switch 19 is in
fact three pairs of SCRs (thyristors), each pair being connected in
parallel-opposed relation for conduction in either direction if the
respective control gates 19G are enabled, only one of those SCRs is
depicted in this view. The three pairs of SCRs are respectively for
each of the 3 phases of power supply. Normally, the control gates
19G are connected in common and controlled in unison. Power on
utility grid bus 10 and/or power on the fuel cell bus 15/15' may
flow through the SCR's 19 when the control gates 19G are enabled,
thereby allowing either source to power the loads 14.
The normal mode is G/C in which the utility grid 10 and the fuel
cells 18 are connected. The module 17 includes circuitry 45 for
sensing when the supply of power from the utility grid bus 10 is
out of limits. Typically, these limits include a voltage and a
current range relative to the standard or nominal values, and the
sensing circuitry 45 provides a signal on lead 47 to control logic
49, and static switch control logic 49A thereof specifically, to
indicate when the grid is outside those limits. The sensing or
detection circuitry 45 is fast acting, providing a response in
about 2 ms. Although not depicted, a separate fast acting frequency
detector may monitor the grid frequency and provide an "in" or
"out" "of limits" signal to the static switch control logic 49A.
"Out of limit" grid signal values include, for example,: a)
instantaneous grid voltage magnitudes, on any phase, outside the
range of 480 v+8% to -15%; b) instantaneous over-current, on any
phase, greater than 2,000 amperes; c) frequency deviations from
nominal 60 Hz value for more than 0.5 sec.; as well as others. The
control logic 49A acts in response to the grid going out of limits,
to provide a signal to the SCR gates 19G to disable them. The SCR's
19 will rapidly commutate off, thereby disconnecting the utility
grid bus 10 from both the loads 14 and the fuel cells 18. A current
sensor 42' senses the current through the SCRs and provides an
indication to the control logic 49A of the occurrence of zero
current through the SCRs. This information is used by the logic 49A
to make the SCR commutation faster. This entire action typically
occurs in about 1/4 cycle (4 ms), thus facilitating a seamless
transfer of power sources from the grid 10 and the fuel cells 18,
to the fuel cells 18 alone, with the fuel cells reconfiguring as
rapidly, as will be explained. This is significantly faster than
the 8 ms or more required to commutate an SCR using conventional
line commutation.
The control logic 49B also uses the voltage and current sensors 37,
41, 42, and 46 to operate the switching gear devices 21, 23, and
23A under various grid, load, and fuel cell out of limit or fault,
conditions. For example, if a load over-current condition exists
such that the current rating of the static switch 19 may be
exceeded, switch 23A is closed to conduct the fault current to the
load 14, by-passing the static switch. As a further example, a fuel
cell fault can be indirectly detected by observing a low load
voltage and perhaps a high grid current but no load over-current.
In such event, switch 21 is opened to isolate the fuel cell fault
from the load 14. The control logic 49A also provides an M1 mode
signal on lead 401 and an M2 mode signal on lead 402. For manual
control from the SSC 29, a G/I status signal is provided by control
logic 49 on lead 403, and a SW19 Enable signal is received on lead
404. The signals 401 and 402 are part of the control signal bus 40,
and the signals on leads 403 and 404 may be conveyed via
communications link 52. When the sensing circuitry 45 senses the
grid to be out of limits, it causes the M2 mode signal on lead 402
to transition from an "Off" to an "On" state to signal a need for,
and to initiate, a mode change. Similarly, but slightly delayed,
when the static switch 19 has actually opened in response to the
sensed out of limits condition of the grid, the M1 mode signal on
lead 401 transitions from an "Off" to an "On" state to signal
nominal completion of the mode change. The reverse occurs when the
sensing circuitry determines that the grid power supply has been
returned to within the acceptable limits, with the M2 signal again
leading the M1 signal.
Referring to FIG. 5, a relevant portion of the SMC 31 and its
control of the fuel cell 18 PCSs is depicted in greater detail,
though it will be understood that the SMC provides additional
control functions such as load sharing and the like, not shown. As
mentioned above, the potential transformer 37, here depicted in the
alternative as a separate transformer 37', is incorporated as part
of the SMC 31. The M1 and M2 signals from the static switch module
17 are inputted to an interface circuit 51, which conditions each
of those signals to provide respective discrete signals D1 and D2
on leads 401' and 402' connected to the PCS portions of each of the
several fuel cell power plants 18 for controlling gating and
sequencing of the inverters (not shown) therein during mode
changes.
Synchronization of the fuel cell power plants 18 in either the G/C
or the G/I mode is effected by a "sync" signal appearing on lead
53. The sync signal is provided through a phase-lock loop 55
receiving alternative inputs, through switch 57, from either a
zero-crossing detector 59 connected to the stepped-down utility
grid bus 10' or an internal frequency source, such as crystal 61. A
"loss of grid" detector 63, similar to circuit 45, is connected to
the stepped-down utility grid bus 10', and provides a control
signal which actuates 3-pole switch 57 as a function of whether or
not the grid power is within limits. The interface circuit 51 also
is responsive to the M1 and M2 mode signals to provide a signal
extended to switch 57 to toggle that switch as a function of the
respective mode. It will be understood that detector 63 might be
omitted and the output of detector circuit 45 from module 17 used
in its stead to control the M1 and M2 mode signals applied to
interface circuit 51, which in turn controls the switch 57. The
switch is depicted in the normal G/C mode in which the
synchronization signal provided to the PCS of the fuel cells 18 is
that of the utility, such that the frequency and phase of the
outputs from the fuel cell inverters are controlled to become and
be, the same as it.
When the system 8 operates in the G/I mode, the frequency and
phasing of the outputs of fuel cells 18 is determined by the
crystal 61. When the utility grid power source returns to within
limits and the system 8 is to be returned to the G/C mode, the
phase and frequency sources are similarly returned. The phase lock
loop 55 slews the sync signal in its transition from one mode to
the other to avoid steps or discontinuities.
The solid state inverters of the PCSs of the respective fuel cell
power plants 18, and the high speed solid state gates (not shown)
which control them, are capable of responding in the 1/2 cycle (4
ms) needed for the seamless transfer of power sources. Thus, these
inverters, through control of their gates by the mode control
signals D1 and D2, are able to effect mode changes of the fuel
cells 18 rapidly enough to accomplish the seamless transfers. This
enables the fuel cell power plants 18 to operate substantially
continuously in a power generating mode, either G/C or G/I, with
but a momentary (less than 4 ms) interruption as they are
reconfigured for operating in the opposite mode. The power
conditioning systems (PCSs), and particularly their inverters and
associated gating logic and control, for each fuel cell power plant
18 are of a type manufactured by Magne Tek Inc. of New Berlin,
Wis.
Reference to the Table depicted in FIG. 6, in combination with the
description of the power system 8 provided above and hereinafter,
will complete an understanding of the invention. During normal G/C
operation, both mode signals M1 and M2, and thus also D1 and D2,
are "Off", the static switch 19 is "On" (conducting/closed), the
inverter gates in the PCSs are enabled, and the sync for the system
8, and particularly the fuel cell PCSs, is provided by the utility
grid bus
Specifically, M2 rapidly transitions to "On", while M1 remains
"Off" for the brief interval required for switch 19 to transition
from "On" to "Off". The discrete signals D1 and D2 have the same
states as M1 and M2, respectively. The transition of signal M2 (and
thus D2) to the "On" state serves to briefly turn "Off" the
inverter gates in the PCSs such that, for a brief interval less
than 4 ms, the PCSs of the fuel cells 18 do not provide an
electrical power output while they are being reconfigured to the
G/I mode of operation. During this interval, the PCS output
regulators are being reconfigured, such that in the G/C mode they
regulate power (real) and VARs and in the G/I mode they regulate
voltage and frequency. The sync is also being reconfigured during
this interval. This interruption is sufficiently brief and the
switch 19 sufficiently fast, that there is little or no chance for
an overload on grid 10 to adversely impact the remainder of power
system 8.
After this brief interval of 4 ms, or less, the system 8 is
reconfigured and operating in the G/I mode. The mode signals M1 and
M2 (and thus also, D1 and D2) are both "On", the switch 19 is "Off"
(open) such that the system is disconnected from the utility grid
bus 8, and the inverter gates in the PCSs are again on to provide
power to the load(s) 14 from the fuel cells 18. At this time, the
output from the PCSs is being "clocked", or synchronized, by the
crystal 61. In the G/I mode, the fuel cell power plants 18 supply,
or continue to supply, power to the loads 14 at regulated voltage
and frequency without involvement of the utility grid, at least to
the maximum capacity of the collective fuel cells.
At such time as the utility grid bus 10 comes back within
acceptable limits as determined by sensor 45, the control logic 49
of the static switch module 17 reverses the prior mode change
sequence and begins the transition from the G/I mode back to the
G/C mode. Mode signal M2 first goes "Off" while M1 briefly remains
"On", the switch 19 quickly transitions from "Off" to "On" such
that the utility grid bus 10 is once again connected to the loads
14 together with the fuel cells 18, the PCS inverter gates are
again briefly "Off" during reconfiguring, and the PCS
synchronization is changing from reliance on crystal 61 to that of
the utility grid supply. The internal PCS output regulation changes
from voltage and frequency to power and VARs. Following the brief
interval (less than 4 ms) for reconfiguring, the system 8 has been
returned to the G/C state, or mode.
Although the invention has been described and illustrated with
respect to the exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made without
departing from the spirit and scope of the invention. For example,
the static switch module 17, and particularly switch 19 therein,
is/are depicted as being separate from and external to, the fuel
cells 18 and their respective PCS's, thus providing the economy of
singular control elements responsible for controlling multiple fuel
cells. However, it will be appreciated that these controls could be
integral with or internal to the respective fuel cells,
particularly if there is but a single fuel cell power plant.
Moreover, although the static switch 19 is described in the context
of pairs of SCRs, it will be appreciated that other static
switching devices capable of similar switching speeds and current
ratings may also be used. It will also be understood that a greater
or lesser number of fuel cell power plants may be employed, and
both the voltage and the current ratings associated with the
elements discussed herein may be greater or less than described.
Similarly, the control circuits described herein as being in the
static switch module 17 could reside in the SMC 31.
* * * * *